In accordance with the prior art, there has already been described certain compounds which are known to be of value as antiparasitic agents. Included among these are such avermectin compounds which are described and claimed by Bishop Bernard Frank et al., in International Publication Number WO 94/15944, and by Fisher, Michael H. et. al., in European Publication Number EP 0 379 341 A2. However, according to the above conventional methods, the yield of the final reaction product was not always satisfactory. It is an object of this invention to convert the avermectin compound of formula (II) to avermectin compound of the formula (I) in high yield.